Farmscale Biochar with Moxham/TFOD

The Moxham, named after Geoff Moxham is a large metal cylinder open both ends. One end is set on the ground and sealed from the outside with soil; the technique is called top fed open draft (TFOD).

The same process can be achieved by digging a hole to suit the quantity of Biochar you intend to make.

For a strong fire we need heat, oxygen and fuel. The TFOD design allows enough oxygen to fuel the flames, but prevents oxygen turning the solid base of charcoal to ash with a barrier of flame. This basic behavior of fire is what allows such a simple technique to efficiently create Biochar without any advanced or expensive technology.

We constantly add fuel when needed which keeps the flame barrier above the smaller pieces of charcoal but not so much as to smother the fire.

The material we use to make Biochar is called feedstock, the feedstock used can be anything biological. We use sustainably managed plantation hardwood, chopped to ~60cm long and split to ~5cmx10cm – Any bigger and it tends to slow down the process. If the feedstock is wet, it is better to split it to a smaller size as the fire needs to drive off most moisture and gas before it will catch and turn to charcoal.

Start a fire in the bottom middle of the TFOD and build up embers with sticks & leaves until the coals are hot enough to light your main feedstock.

The feedstock should be fed at a pace which keeps the flames strong. Be careful of smothering the flames or letting them smolder. Strong flames burn off the gasses released from the wood, which keeps pollution at a minimum and entering oxygen cycles any excess smoke back into the flame which further reduces pollution.

The colour and amount of smoke can help to understand the state of the fire. If the smoke is thick and almost yellow, the fire has been left too long and it’s starting to smolder. If the smoke is thick and white, you have smothered it and need to wait for it to dry and catch onto the wood. When there is very little to no smoke, it is working efficiently and cleanly!

Once filled to the amount needed, we tip the Moxham with a tractor and spread the Biochar over sheet metal while hosing with a very generous amount of water to kill any hot spots. If hot coals are missed and left unattended, you may lose your Biochar to ash. If the TFOD was designed as a hole in the earth, a much greater amount of water is needed to kill any heat. If this design isn’t so big, the Biochar could be shoveled out and then cooled with water.The purpose of cooling the Biochar quickly with water is to cause it to become hydrophilic (absorbing water). If it is covered and left to cool for a few days as is done when making charcoal for heaters .etc – the Biochar will become hydrophobic and be not as good for soil addition. Although it is possible to condition Biochar at any time, it just takes a while longer to condition when dry and hydrophobic.

The finer your Biochar, the better for soil addition. We achieve this with a tractor powered grinder and then grade through a ~5mm x 5mm screen.